1. Field of the Invention
The invention relates to the construction of marine vessels carry liquid cargos such as very large crude oil carriers (“VLCC”), and specifically to the requirements for providing cargo expansion space during transit.
2. Description of Related Art
The International Convention for the Prevention of Marine Pollution from Land-Based Sources (MARPOL) design requirements for marine vessels have substantially reduced the actual cubic capacity of tankers, in some cases by as much as 20%-30%. For example, tanker capacity has been reduced by 2% to 3% as a result of the double hull requirement of MARPOL Annex 1, 13F & G and by 2% as a result of the MARPOL regulation requiring a 2% headspace in each tank of a vessel for heat expansion. MARPOL sets numerous standards for such vessels, one being the standard for crude oil washing commonly referred to as “COW”.
While oil is typically sold in volume, tankers earn freight by the amount of liquid cargo they carry in weight. Thus, if a ship's internal volume, (referred to as “cubic capacity” or simply “capacity”) is increased, the ship can earn increased freight income by the amount of weight the volume can accommodate. The increase in income can be calculated as cubic meters times specific gravity of cargo times freight rate in dollars.
Numerous patents address venting hydrocarbon gases and overflow prevention systems, but do not address problems and solutions relative to the reduction of cubic capacity in ships as a result of MARPOL regulations. In U.S. Pat. No. 3,851,611 to Yamamoto, a tank for low temperature liquefied gas tanker ship is described wherein the tank has a flexible membranous lower portion below the hull and a rigid, but flexibly attached upper portion above the hull deck. A fluid tight connection between the portions is maintained through the deck.
The tank in Yamamoto, including the upper portion, is filled with liquefied gas to enhance the stability of the overall ship and provide more room for the liquefied gas. The upper portion provides enhanced stability because its reduced horizontal cross-sectional area has a smaller free surface area that is further enhanced by partitions. Yamamoto, however, increases ship instability by raising the center of gravity of the liquefied gas through the filling of the upper tank positioned above the deck and allowing for the flexible shifting of the rigid upper central portion relative to the lower portion of the tank. Yamamoto has the upper portion of the tank centrally positioned above the membranous lower tank, but does not address any reason or any advantages for the positioning of the upper tank.
In U.S. Pat. No. 4,144,829 to Conway discloses a system for the venting of hydrocarbon gasses including utilizing the ship's existing pressure/vacuum relief valved venting system and expansion trunk (19) which opens downwardly into the cargo compartment to which it is coupled. See col. 3, lines 40-60. Conway '829, however, connects adjacent cargo compartments with valves 20 and pipes 21 and does not discuss any advantageous location of the expansion trunk or the volume enclosed by the expansion trunk.
In U.S. Pat. No. 4,292,909 to Conway, a trunk line is disclosed placing cargo expansion trunks (16) in fluid communication with a retention tank (21) configured for receiving any spill overflow during the loading of petroleum products. Thus, Conway '909 interconnects trunks (16) as part of an overflow prevention system and does not address advantageous location of the trunks (16), the volume enclosed by the trunks (16), or the integration of pipelines for crude oil washing.
In U.S. Pat. No. 4,233,922 to Conway, a fluid transfer system for vessels is disclosed for containing and transferring of contaminated gasses formed by fluid chemical and petroleum products. The fluid transfer system includes placing a plurality of cargo expansion trunks coupled with and opening downwardly into each cargo compartment which are in fluid communication with a branch vent line and a longitudinally disposed trunk vent line. While Conway '922 discusses in detail the use of the trunks as conduits, it does not address the size of the expansion trunk in relation to MARPOL expansion requirements, any advantageous location of the expansion trunk, or any integration of the requirement for crude oil washing.
A need exists for an improved carrying capacity for maritime tankers that can meet MARPOL requirements for the 2% headroom expansion using a gas expansion trunk located above deck and be in compliance with other MARPOL requirements, such as crude oil washing.